1. Field of the Invention
The present invention relates to an apparatus and a method for measuring each thickness of a multilayer stacked on a substrate, e.g., a semiconductor wafer. More particularly, the present invention relates to an apparatus and method for directly measuring a thickness of the multilayer using a patterned wafer rather than a monitoring wafer.
2. Description of the Related Art
Generally, a semiconductor fabricating process includes a deposition process for coating a thin layer, such as an insulation layer, a dielectric layer and a metal layer, on a wafer of monocrystalline silicon, and a photolithography process for forming a predetermined pattern on the thin layer. In addition, the thin layer is usually measured to confirm whether the thin layer is coated to a desired thickness and is etched away after performing the deposition or the photolithography processes.
Conventionally, a thickness of a thin layer is measured using a monitoring wafer, which is a kind of specimen wafer. When some process in the fabrication of a semiconductor device is performed, the monitoring wafer is subjected to the same process as a working wafer, which will be subsequently referred to as a patterned wafer after completion of the process. Then, the thickness of the layer on the monitoring wafer is measured and a process failure is detected based on the measured thickness of the layer. The layer on the monitoring wafer may be a layer that is newly formed by the process or a residual layer remaining thereon after the process.
The monitoring wafer only includes a single layer coated in the previous deposition process or a residual layer remaining after a previous etching process, so that a lower layer disposed under the single layer or the residual layer may not be formed in the same process. Accordingly, when the monitoring wafer is used for the measuring process, the thickness of only a single layer is measured. However, various kinds of thin layers are coated on the working wafer, and therefore, a plurality of monitoring wafers corresponding to each thin layer on the working wafer is necessary to measure each thickness of a multilayer on the patterned wafer. Furthermore, the monitoring wafer is usually discarded after only one or two measuring processes to maintain accurate measurement of the thickness. As a result, cost for the measuring process using the monitoring wafer is very high. In addition, there is a problem in that the thickness of the thin layer on the monitoring wafer is not always identical to the thickness of the thin layer on the patterned wafer.
Accordingly, the thickness of the thin layer requires measurement directly using the patterned wafer rather than the monitoring wafer. In general, various kinds of layers are sequentially stacked on the patterned wafer, and the layers as a whole stacked on the patterned wafer are collectively referred to as a multilayer. A multilayer may be referred to as a variable multilayer in that some of the stacked layers may be removed or other layers may be additionally stacked on the multilayer in a subsequent process. Therefore, the measuring method of the thickness of the thin layer using the patterned wafer has a fundamental problem in that the thickness of the layer needs to be measured without destroying the variable multilayer to prevent the patterned wafer from being damaged. Conventionally, a dual beam spectrometry method or a spectroscopic ellipsometry method has been used to measure the thickness of the layer without destruction thereof.
In the dual beam spectrometry method, light is incident on the patterned wafer at a substantially right angle from a light source, and a reflected light reflected from the wafer is divided into a sample channel and a reference channel. An intensity of the incident light is measured, and a reference intensity is calculated using a silicon reference chip. A relative reflectivity is obtained from the reference intensity and the actual intensity of the reference channel. Then, a thickness of the layer is obtained using the relative reflectivity. However, the dual beam spectrometry method has a problem in that the measured thickness is not accurate in a case where the layer is very thin or has a multilayer structure in which a plurality of layers is stacked on the wafer.
In the spectroscopic ellipsometry method, polarized light is incident on the wafer at a predetermined angle from a light source, and a reflected light reflected from the wafer is divided into horizontal and vertical polarization components with respect to the polarization direction. Then, a light intensity ratio of the horizontal and vertical polarization components is calculated, and a phase difference between the horizontal and vertical polarization components is obtained. A thickness of a layer on the wafer is obtained using the light intensity ratio and the phase difference between the horizontal and vertical polarization components.
According to the dual beam spectrometry method and the spectroscopic ellipsometry method, the reflected light reflected from the wafer is divided in accordance with a wavelength of the light, and a spectrum of each wavelength is measured. Then, the measured spectrum is compared with a theoretical spectrum, and a theoretical thickness corresponding to the theoretical spectrum that is substantially similar to the measured spectrum is determined to be the thickness of the layer.
When a multilayer is formed on the patterned wafer, the theoretical spectrum is accurately calculated on a condition that structural information of the multilayer, e.g., a stacked structure and a material characteristic of each component layer, is fully known. Specifically, the theoretical spectrum is significantly influenced by a type of each component layer, a sequential order along which the component layer is stacked, a refractive index n of each component layer, and an extinction coefficient k of each component layer. The refractive index is defined as a ratio of a velocity of light in the layer to the velocity of light in a vacuum. The extinction coefficient is defined as a reducing rate of the intensity of light when light passes through the layer.
Therefore, when the information on the multilayer is not accurate, the measured spectrum may not be identical to the theoretical spectrum, and the measured thickness of the multilayer may not be reliable.
In a theoretical patterned wafer, each component layer of the multilayer is stacked on the wafer according to a designed sequential order, and the structural information of the multilayer is well known. However, when a process failure is generated during formation of one of the component layers in the multilayer, and the actual stacked structure of the multilayer is different from an expected structure of the multilayer, the structural information of the multilayer may not be accurately known. As a result, the thickness of the component layer is not accurately measured by the dual beam spectrometry method or the spectroscopic ellipsometry method. In particular, when the multilayer is formed to have a locally different stack structure due to a poor evenness of the patterned wafer, the measured thickness of a component layer in the multilayer is completely unreliable.